1. Field of the Invention
The present invention relates to an image reading apparatus for reading by means of a read sensor an image on the original exposed to light by an irradiation lamp, and more particularly to an image reading apparatus so designed as to prevent the deterioration of reading characteristics due to variation in the temperature of a read sensor.
2. Description of the Related Art
In image reading apparatus, an image on an original is read out by using an irradiation lamp to expose the original to light and to cause the light reflected from the original to focus on the imaging face of a sensor. Incidentally, image reading in this specification includes reading a white reference plate for shading correction, reading by pre-scanning for deciding the size of an original and whether the original is monochromatic or color and so forth in addition to the operation of reading an image on the original.
In this image reading apparatus, the irradiation lamp is turned on during the time an image is read. In consequence, the irradiation lamp generates heat during that time, which raises the temperature in the casing of the image reading apparatus and also causes the temperature of the read sensor to rise. A curve P of FIG. 27 shows variation in the temperature of the irradiation lamp.
Moreover, a CCD (Charge Coupled Device) image sensor is generally employed as the read sensor. During the image reading operation, a sensor driving circuit for driving the CCD image sensor is actuated and a clock signal is supplied from the sensor driving circuit. Therefore, the read sensor generates heat more than when it is on standby and raises its own temperature.
When the temperature of the read sensor rises, there may arise various problems: the sensitivity of the read sensor varies; the focal position of an image forming system shifts as members constituting the read sensor become deformed by thermal expansion; and the read position changes.
Accordingly, it has been a common practice to cool the irradiation lamp for the original and the read sensor by installing a cooling mechanism in the casing of the image reading apparatus. In order to simplify such a cooling mechanism in this case, one cooling fan is normally charged with cooling both the irradiation lamp and the read sensor during the image reading operation. With the cooling mechanism, the temperature of each of the members including the read sensor is kept remaining close to a predetermined temperature level during the image reading operation.
In the case where the read sensor is driven only during the image reading operation as noted previously, the reading characteristics of the read sensor may become unstable immediately after electric power is supplied thereto, that is, while the read sensor remains in low-temperature condition. Therefore, the driving voltage is always applied to the read sensor to obviate the instability of the reading characteristics of the read sensor immediately after the power is supplied thereto. Notwithstanding, the temperature of the read sensor still continue rising gradually even during the time no images are read, that is, in the standby condition.
In the conventional image reading apparatus, the cooling fan is rotated during the image reading operation to cool the read sensor as well as the irradiation lamp. Since the driving voltage is applied to the read sensor even in the standby condition as set forth above, the temperature of the read sensor rises because of the self-generation of heat as shown by the curve Q of FIG. 27, the read sensor together with the irradiation lamp is cooled when the reading operation is started and its temperature lowers and settles down to a constant temperature T.sub.1 a predetermined time t.sub.1 after.
On the other hand, parameters for use in forming images in the image reading apparatus are usually set in an area S where temperatures are stabilized. Thus the sensitivity of the read sensor and the configuration of the read-sensor fitting member are caused to vary until the time t.sub.1 (sec) is stabilized. Even when the same original is read, the image data varies with the lapse of time in the course of processing the image by subjecting the image signal thus read to A/D conversion.
In the case of a monochromatic image reading apparatus, any readout image is set free from being badly affected even though the sensitivity of a read sensor varies because the output of the read sensor is normally subjected to a black-and-white binary process.
In contrast with the monochromatic image reading operation, colors are each read by multiple gradations, for example, 256 gradations in a color image reading apparatus, whereby variations in the sensitivity of the read sensor appear as variations in the gradation of an image to be read. Particularly in the color image reading apparatus, the tone of the image varies with the change of gradation of each color.
In other words, as shown by the curve Q of FIG. 27, the temperature as a basis in setting each parameter may differ from the actual temperature during the period t.sub.1 (sec) from the start of the image reading operation until the temperature of the read sensor settles down to the predetermined temperature T.sub.1. Consequently, there arises inconvenience in that an image different in coloring from the original image is read for the duration of time t.sub.1 (sec).